SpaceX chairman Elon Musk said the company's Dragon capsule, now used to ferry cargo to the international space station, should be ready to carry astronauts to and from space within two or three years, and that he is more optimistic than ever that a partially reusable rocket will accelerate the reduction in launch costs that SpaceX has already caused with its Falcon 9.

Speaking at the All Things Digital conference in Rancho Palos Verdes, Calif., Musk said his ultimate goal with Hawthorne, Calif.-based Space Exploration Technologies Corp. (SpaceX) has not changed since he founded the company: placing humans on Mars to start a permanent colony there.